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The word avoirdupois is from Anglo-Norman Frenchaveir de peis (later avoir de pois), literally "goods of weight" (Old French aveir, "property, goods", also "to have", comes from the Latinhabere, "to have, to hold, to possess property"; de = "from"/"of", cf. Latin; peis = "weight", from Latin pensum).[2][3] This term originally referred to a class of merchandise: aveir de peis, "goods of weight", things that were sold in bulk and were weighed on large steelyards or balances. Only later did it become identified with a particular system of units used to weigh such merchandise. The orthography of the day has left many variants of the term, such as haberty-poie and haber de peyse. (The Normanpeis became the Parisianpois. In the 17th century de was replaced with du.)[4]

Comparison of the relative sizes of avoirdupois, Troy, tower, merchant and London pounds.

There are two major hypotheses regarding the origins of the avoirdupois system. The older hypothesis is that it originated in France.[5] A newer hypothesis is that it is based on the weight system of Florence.[6]

The avoirdupois weight system is thought to have come into use in England circa 1300. It was originally used for weighing wool. In the early 14th century several other specialized weight systems were used, including the weight system of the Hanseatic League with a 16-ounce pound of 7003720000000000000♠7200 grains and an 8-ounce mark. However, the main weight system, used for coinage and for everyday use, was based on the 12-ounce tower pound of 7003540000000000000♠5400 grains. From the 14th century until the late 16th century, the avoirdupois pound was also known as the wool pound or the avoirdupois wool pound.

The earliest known version of the avoirdupois weight system had the following units: a pound of 7003699200000000000♠6992 grains, a stone of 14 pounds, a woolsack of 26 stone, an ounce of 1⁄16 pound, and finally, the ounce was divided into 16 "parts".[7]

The earliest known occurrence of the word "avoirdupois" (or some variant thereof) in England is from a document entitled Tractatus de Ponderibus et Mensuris or Treatise on Weights and Measures. This document is listed in early statute books under the heading 31 Edward I dated 2 February 1303. More recent statute books list it under Statutes of uncertain date. Scholars nowadays believe that it was probably written between 1266 and 1303.[8] Initially a royal memorandum, it eventually took on the force of law and was recognized as a statute by King Henry VIII and Queen Elizabeth I. It was repealed by the Weights and Measures Act of 1824. In the Tractatus, the word "avoirdupois" refers not a weight system, but to a class of goods, specifically, heavy goods sold by weight, as opposed to goods sold by volume, by count, or by some other method. Since it is written in Anglo-Norman French, this document is not the first occurrence of the word in the English language.[9][10]

Three major developments occurred during the reign of Edward III (r. 1327-77). First, a statute known as 14o Edward III. st. 1. Cap. 12 (1340) "Bushels and Weights shall be made and sent into every County."[11]

English translation: "(4) it is assented and accorded, That from henceforth one Measure and one Weight shall be throughout the Realm of England; (5) and that the Treasurer cause to be made certain Standards of Bushels, Gallons, of Weights of Auncel, and send the same into every County where such Standards be not sent before this Time;"

The second major development is the statute 25o Edward III. st. 5. Cap. 9. (1350) "The Auncel Weight shall be put out, and Weighing shall be by equal Balance."[12]

English translation: "so that the Sack of Wooll weigh no more but xxvi. Stones, and every Stone to weigh xiv. l."

The third development is a set of 14th-century bronze weights at the Westgate Museum in Winchester, England. The weights are in denominations of 7 pounds (corresponding to a unit known as the clip or wool-clip), 14 pounds (stone), 56 pounds (4 stone) and 91 pounds (1⁄4 sack or woolsack).[13][14] The 91-pound weight is thought to have been commissioned by Edward III in conjunction with the statute of 1350, while the other weights are thought to have been commissioned in conjunction with the statutes of 1340. The 56-pound weight was used as a reference standard as late as 1588.[15][16]

In 1588 Queen Elizabeth increased the weight of the avoirdupois pound to 7003700000000000000♠7000 grains and added the troy grain to the avoirdupois weight system. Prior to 1588, the "part" (1⁄16) was the smallest unit in the avoirdupois weight system. In the 18th century, the "part" was renamed "drachm".

In the United Kingdom, 14 avoirdupois pounds equals one stone. The quarter, hundredweight, and ton equal respectively, 28 lb, 112 lb, and 2,240 lb in order for masses to be easily converted between them and stones. The following are the units in the British or imperial version of the avoirdupois system:

Table of mass units

Unit

Relative
value

Metric
value

Notes

dram or drachm (dr)

1⁄256

≈ 1.772 g

1⁄16 oz

ounce (oz)

1⁄16

≈ 28.35 g

16 dr

pound (lb)

1

≈ 453.6 g

16 oz

stone (st)

14

≈ 6.350 kg

1⁄2 qr

quarter (qr)

28

≈ 12.70 kg

2 st

hundredweight (cwt)

112

≈ 50.80 kg

4 qr

ton (t)
or
long ton

7003224000000000000♠2240

≈ 1016 kg

20 cwt

Note: The plural form of the unit stone is either stone or stones, but stone is most frequently used.

The 13 British colonies in North America used the avoirdupois system. But they continued to use the British system as it was, without the evolution that was occurring in Britain in the use of the stone unit. In 1824 there was landmark new weights and measures legislation in the United Kingdom that the United States did not adopt.

In the United States, quarters, hundredweights, and tons remain defined as 25, 100, and 7002907184740000000♠2000 lb respectively. The quarter is now virtually unused, as is the hundredweight outside of agriculture and commodities. If disambiguation is required, then they are referred to as the smaller "short" units in the United States, as opposed to the larger British "long" units. Grains are used worldwide for measuring gunpowder and smokeless powder charges. Historically, the dram has also been used worldwide for measuring gunpowder charges, for measuring powder charges for shotguns and large black-powder rifles.

In the avoirdupois system, all units are multiples or fractions of the pound, which is now defined as 6999453592370000000♠0.45359237kg exactly in most of the English-speaking world since the implementation of the international yard and pound agreement of 1959.[17]

Due to the ambiguous meanings of "weight" as referring to both mass and force, it is sometimes erroneously asserted that the pound is only a unit of force. However, as defined above the pound is a unit of mass, which agrees with common usage. Also see pound-force, poundal and pound-mass.